Monolithic concrete beam connection.

[KingKongdoor]

I have a cantilevered concrete beam that supports a perimeter concrete beam and column next to an adjacent foundation. I cant find a good example to reference back to for this condition. Attached is a concept of how I was thinking it needs to be done. Just wondering what people's thoughts are on this concept. I'm currently sizing stirrups for the shear and making sure the top bars are fully developed into the perimeter beam. The beams will be poured monolithicly. Thanks for any input.

 

[KootK]

Yeah, you [pretty much nailed it. I'd run both the top and bottom bars of the cantilever inside of the transverse grade beam longitudinal bars. Do it for the top bars to eliminate a potential plane of weakness with no reinforcing. Do it for the bottom bars just because it looks right and is conventional. I don't think that any of this is really critical, however, given the loading situation.

A couple of other housekeeping items that you may well have already considered:

1) There is a demand for top steel anchorage in the cantilever tip that is analogous to bottom steel anchorage at the support in simple span beams. That might affect your bar sizes and/or anchorage detailing.

2) You may need hanger steel in the cantilever end to transfer shear from the grade beams tying in. Your tie-in loads may well be small enough that it's not an issue.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

If the cantilever span is short enough, you might begin approaching a corbel type behavior vs. a pure beam cantilever.
If so - then some additional Ah steel might be needed on the sides near the top of the cantilever. Just a thought.

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[KingKongdoor]

I concur with the cantilever bars being enclosed by the perimeter beams longitudinal steel.

1) There is a demand for top steel anchorage in the cantilever tip that is analogous to bottom steel anchorage at the support in simple span beams. That might affect your bar sizes and/or anchorage detailing.

This plus the anchor bolts most likely needing reinforcement can get messy.​

2) You may need hanger steel in the cantilever end to transfer shear from the grade beams tying in. Your tie-in loads may well be small enough that it's not an issue.

By hanger steel i assume you mean stirrups at an angle that tie perimeter beam to cantilever across the shear plane. This is where most of my concern was if the concrete depth did not provide adequate shear strength. I typically like to just install vertical stirrups because I don't trust contractors to install the stirrups properly otherwise. A vertical stirrup would still reinforce the shear plane but would not be in the direct path of internal forces. Do you feel that vertical stirrups placed per ACI would not provide proper shear reinforcement in this condition?​

 

[KootK]

By hanger steel i assume you mean stirrups at an angle that tie perimeter beam to cantilever across the shear plane.

I do it as vertical stirrups in the cantilever tip, right behind the grade beams. It may not even be necessary if the grade beam loads are minor (curtain wall load etc).

A vertical stirrup would still reinforce the shear plane but would not be in the direct path of internal forces.

I don't see it. You may need to elaborate or provide a sketch.

Do you feel that vertical stirrups placed per ACI would not provide proper shear reinforcement in this condition?

I do indeed. It's more congestion at the beam intersection but manageable in my opinion.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KingKongdoor]

Quote (KingKongdoor)
[highlight #4E9A06]Do you feel that vertical stirrups placed per ACI would not provide proper shear reinforcement in this condition?[/highlight]

(KootK)I do indeed. It's more congestion at the beam intersection but manageable in my opinion.



I concur with your first statement "I do it as vertical stirrups in the cantilever tip, right behind the grade beams. It may not even be necessary if the grade beam loads are minor (curtain wall load etc)." but the reply above reads to me like you don't think vertical stirrups provide shear reinforcement in this case.

 

[KootK]

Yeah, I missed the "not" in the green statement. Vertical stirrups for me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.